Method for improving the brightness of kaolin clay

ABSTRACT

An improved process for improving the brightness of impure kaolin clay containing discrete colored particles of titaniferous impurities wherein the impurities are selectively floated from a dispersed slip of the clay by quiescent (rather than by dynamic) means and the purified clay is further brightened by treatment with a reducing bleach reagent such as zinc hydrosulfite. In carrying out such process, the time required for conditioning the slip of impure clay with an anionic reagent selective to the colored impurities, and consequently the power demand required to obtain a product of desired quality by quiescent flotation, are reduced significantly by subjecting the slip of beneficiated clay, after quiescent flotation of colored impurities, to a mild oxidation treatment before bleaching the purified clay.

United States Patent Mallary et al.

[ Jan. 21, 1975 METHOD FOR IMPROVING THE BRIGHTNESS OF KAOLIN CLAY [75] Inventors: Miller B. Mallary; Joseph L. Hunter, both of Macon, Ga.

[73] Assignee: Engelhard Minerals & Chemicals Corporation, Township of Woodbridge, NJ.

[22] Filed: June 13, 1972 [21] Appl. No.: 262,265

[52] US. Cl. 106/288 B, 209/5 [51] Int. Cl C08h 17/06 [58] Field of Search 106/288 B, 72; 209/5; 423/118 [56] References Cited UNITED STATES PATENTS 3,353,668 11/1967 Duke 106/288 B 3,489,586 1/1970 Chapman et al. 106/288 B 3,616,900 11/1971 Cecil et al .1 106/72 3,670,883 6/1972 Weir 209/5 Primary Examiner-Delbert E. Gantz Assistant Examiner]ames W. Hellwege Attorney, Agent, or FirmMelvin Cv Flint; lnez L. Moselle [57] ABSTRACT An improved process for improving the brightness of impure kaolin clay containing discrete colored particles of titaniferous impurities wherein the impurities are selectively floated from a dispersed slip of the clay by quiescent (rather than by dynamic) means and the purified clay is further brightened by treatment with a reducing bleach reagent such as zinc hydrosulfite In carrying out such process, the time required for conditioning the slip of impure clay with an anionic reagent selective to the colored impurities, and consequently the power demand required to obtain a product of desired quality by quiescent flotation, are reduced significantly by subjecting the slip of beneficiated clay, after quiescent flotation of colored impurities, to a mild oxidation treatment before bleaching the purified clay.

4 Claims, N0 Drawings METHOD FOR IMPROVING THE BRIGHTNESS OF KAOLIN CLAY BACKGROUND OF THE INVENTION Colored titaniferous impurities may be removed from kaolin clay by froth flotation with an anionic collector for the titania. A commercial process, known as Ultraflotation, is described in U.S. Pat. No. 2,990,958 to Greene et al. This process features the use of a collector coated carrier solid, such as fatty acid coated calcite, to aid in the collection of the reagentized colored titania. In carrying out the process, an alkaline deflocculated slip of the clay, preferably a fine size fraction of the clay, is initially conditioned with the carrier, anionic collector and oil. The slip is diluted prior to flotation to avoid clay entrainment in the froth during the flotation steps. The flotation is carried out by aerating and agitating the conditioned slip while adding water to maintain an adequate pulp level so that the impurityladen froth, which also contains the carrier, can overflow into the launderers. The slip of purified clay is normally cleaned several times by reflotation with aeration. The purified clay is recovered as a dilute slip, typically at a solids below percent. Because of the dilution involved in various steps of the process, relatively large volumes of equipment are required to purify kaolin clay by dynamic flotation.

The suggestion has been made to separate reagentized colored titania from a suitable conditioned clay pulp by quiescent means, whereby the impurity forms a floating scum rather than the froth which is obtained by dynamic flotation. Reference is made to Ser. No. 800,281, filed Feb. 18, 1969, now U.S. Pat. No. 3,670,883, by Samuel R. Weir. A potential advantage of quiescent flotation is the significant saving in equipment volume which may be realized because quiescent flotation can be used with clay slips having concentrations such that excessive entrainment of clay would result if the separation were to be carried out by froth flotation. It was also found that separation of titania from fine clays could be effected in the quiescent process without using a carrier. However, when the carrier was omitted in the quiescent process, relatively long conditioning times and consequently large power demands were required to obtain desired product quality. The power input was substantially greater than was required for Ultraflotation.

In order to provide adequate conditioning time and power input in commercial scale equipment for the quiescent process, it was evident that larger conditioning volumes would be required to obtain desired product quality. Consequently, the practical commercial utilization of the quiescent flotation principle without use of a carrier has awaited the discovery of means for reducing the unusually high energy input normally required to obtain optimum removal of colored impurities by the no-carrier quiescent flotation process.

THE INVENTION Accordingly, a general object of the invention is to provide an economically feasible method for brightening clay by quiescent flotation.

A more specific object is to reduce the power cost and equipment volume required to obtain acceptable beneficiated clay products by quiescent flotation.

Stated in another manner, an object is to provide a simple means for minimizing or overcoming the normally adverse effects of using moderate conditioning times and power inputs in the quiescent flotation process for purifying kaolin clay.

A particular object is to provide an economically feasible process for purifying a whole (unfractionated) soft kaolin clay crude whereby purified fine and coarse size fractions of beneficiated clay may be produced.

The essence of the subject invention resides in substituting a post-flotation oxidation step for high energy input during the conditioning step in a method for beneficiating a slip of kaolin clay by quiescent selective flotation of titaniferous impurities with a small amount of an anionic collector and hydrocarbon liquid and bleaching the oxidized slip of beneficiated clay with a reducing agent.

We have discovered unexpectedly that a substantial reduction in power input during conditioning may be effected, without adverse effect on finished product quality, if the slip of purified clay obtained by nocarrier quiescent flotation is subjected to an oxidation treatment which may be mild. Thus, practice of this invention permits a substantial reduction in power costs. Furthermore, conditioning time required to impart the power to the slip is reduced. Such reduction in time permits a reduction in the volume of equipment required to condition a desired volume of clay feed.

Surprisingly, benefits of the invention are realized in spite of the fact that the beneficiated clay in the slip, prior to bleaching with a reducing agent, is normally considerably less bright with the reduced power input. For example, we have found that in typical operations of our process, bleached beneficiated clay products having desired high brightness, usually above 90 percent, could be produced at a percent reduction in power input merely by subjecting the beneficiated clay to a mild oxidation treatment before bleaching the clay. It should be pointed out that the reduced conditioning time and power input results in a beneficiated clay which is normally significantly less bright, prior to bleaching with a reducing agent, than the beneficiated clay would be if longer conditioning time and more power were employed. Furthermore, with conventional (soft) kaolins the oxidation treatment does not brighten the beneficiated clay significantly. Thus, the benefit of the post-conditioning oxidation treatment is generally not observable even when a colorless oxidant is employed until after the beneficiated clay has undergone a finishing reduction bleach. The exception of this would be a specific type of clay, such as Georgia hard gray kaolin, which is normally brightened significantly by oxidation.

An unexpected benefit of the invention is that beneficiated clay having exceptionally high brightness at rather high residual titania levels may be obtained. For example, we have produced products having brightness values above 91 percent with 1 percent residual TiO Normally clays with such a high residual TiO analysis have brightness values below percent when colored impurities are removed by froth flotation.

The reasons for the remarkable effectiveness of our post-conditioning oxidation treatment are not fully understood. One possible explanation might be that the oxidation treatment destroys residual flotation reagents in the slip of purified clay. This possible simple explanation is contraverted by the observation that an oxidation treatment which is effective in carrying out the present invention produces no visual changes in the organic reagents when the oxidation is carried out in an acidified water blank. Further indication that destruction of flotation reagents is not per se the explanation is the fact that the beneficiated clay is usually not improved in brightness after oxidation treatment but requires subsequent bleaching with a reducing agent to realize the benefit of the oxidation step.

In a preferred embodiment of the invention, the clay which is treated is a whole (unfractionated) crude soft sedimentary kaolin. Normally grit (e.g., plus 325 mesh matter) is removed from a slip of the crude whole soft clay prior to conditioning, etc., and the slip of purified clay is fractionated after quiescent flotation and oxidation treatment to produce one or more fine fractions (No. 1 and/or No. 2 grade coating clay) and coarse fractions useful as a filler or as feed for a grinding mill. By applying the process to an unfractionated crude, beneficiated coarse clay is obtained in addition to beneficiated fine coating clays. This result cannot be obtained by froth flotation on a commercial scale for the reason, among others, that the beneficiated clay slips obtained by froth flotation are too dilute to undergo fractionation in centrifuges.

PRIOR ART Ser. No. 800,281, filed Feb. 18, 1969, now US. Pat. No. 3,670,883, by Samuel R. Weir, discloses the concept of beneficiating impure kaolin clay by quiescent flotation.

The following patents relate to bleaching slips of specific types of kaolin clay, namely gray Georgia kaolin clay, by oxidants or oxidants followed by reducing agents:

US. Pat. No. 3,353,668 Duke US Pat. No. 3,616,900 Cecil & Jacobs US. Pat. No. 3,635,744 Malden As described in these patents, the oxidation may be carried out with unfloated clay or it may be applied to clay previously beneficiated by froth flotation. In contrast to such prior use of oxidants to bleach constituents in clay, we utilize an oxidant to reduce power input during a beneficiation step. The benefit attributable to the use of the oxidant in our process is unrelated to the use of an oxidant to bleach certain clay. This is evidenced by the fact that the oxidant functions effectively in our process with clays which do not respond to bleaching by a strong oxidant and are no brighter when subjected to oxidation prior to the reduction bleach.

DESCRIPTION OF THE INVENTION The method of the present invention is applicable to the treatment of any naturally-occurring sedimentary kaolin clay containing a particulate colored titaniferous impurity and kaolin particles of various sizes. Particularly good results have been obtained with soft unfractionated sedimentary kaolin clays of the type mined in Georgia.

The clay which is employed as a starting material may be one which has been processed only to the extent of crushing and pulverizing the crude ore as mined. Such clay would be blunged in water and, in most cases, degritted to remove very coarse (e.g., plus 325 mesh) impurities and agglomerates before being deflocculated. It is within the scope of the invention to employ a crude clay which has undergong preliminary treatment such as extrusion, high shear viscous kneading or the like. The term crude as employed herein refers to an unfractionated kaolin clay but encompasses degritted clays or clays in which the particle size distribution of the kaolin particles has been altered by extrusion or the like.

Typical degritted soft Georgia kaolin clay crudes have an average particle size within the range of about 1.5 to 2.5 microns and are composed of clay particles of widely varying size, ranging from a fraction of a micron up to 10 microns or more. Degritted crudes of this type usually analyze between about 1.5 percent to 2.0 percent TiO (All particle sizes in the micron size range refer to values obtained by the Casagrande sedimentation method.) When processing such crudes by the method of this invention, it is generally desirable to reduce the TiO content to about 1 percent or below and to fractionate the purified clay into a purified fine size fraction containing from percent to percent or more by weight of particles finer than 2 microns and a purified coarse size fraction amenable to subsequent grinding or delamination, if desired. Hard Georgia kaolin crudes are composed of finer kaolin particles than soft crudes. Degritted hard clay usually has an average particle size below 1 micron. When processing such clay, the fractionation may be carried out, for example, to produce a fine size fraction essentially percent finer than 2 or 3 microns and 90 percent or more finer than 1 micron.

Conventional means may be employed to blunge and degrit the clay ore. A dispersing (deflocculating) agent, preferably soda ash, sodium silicate, or a mixture thereof, is usually incorporated with the blunged clay before it is degritted at clay solids levels up to about 70 percent. Clay solids are calculated by dividing the dry clay weight by the total weight of clay and water in the system, including the moisture associated with the clay. Dry weight refers to the weight of the clay after being heated to essentially constant weight at 220F.

The quantity of deflocculating agent required to obtain optimum collection of titania after the conditioning step may exceed the amount incorporated in the blunged clay prior to degritting. In such case, additional sodium silicate or a mixture of soda ash and sodium silicate may be added to the pulp of degritted clay. The sodium silicate may be incorporated in the form of a dilute hydrosol as described in US. Pat. No. 3,337,048 to Mercade. It is also within the scope of the invention to use ammonium hydroxide as the sole deflocculating agent or to employ it in combination with other dispersants such as those mentioned above. Suitable proportions of deflocculating agent vary from crude to crude. The illustrative examples describe quantities which have been satisfactory with representative samples of soft Georgia kaolin crudes. After addition of the deflocculating agent, the clay-water systern should be agitated to assure dispersion of all of the particles including the particles of titaniferous impurities.

The deflocculated slips of clay which are charged to the conditioners must be sufficiently fluid to minimize clay entrainment during quiescent flotation. However, the slips should be sufficiently high in clay solids to avoid power wastage during conditioning. Furthermore, the use of relatively high solids clay slips reduces the volume of equipment and the demands on thickeners, filters or dryers which are normally used in subsequent processing. When handling crudes which have not undergone particle size fractionation, the slip of beneficiated clay, which is usually recovered at essentially the same solids as the charge to the conditioners, should be sufficiently high in solids to undergo fractionation in a centrifuge. Thus, we prefer to charge the conditioners with deflocculated slips of degritted clay at a solids level of at least percent. Solids levels within the range of 30 percent to 45 percent are especially recommended. At clay solids levels above 45 percent the slips may be undesirably viscous. Some clays, however, can be prepared into satisfactory fluid slips containing up to 70 percent solids. Any agitated equipment may be used to prepare the high solids deflocculated clay slips or pulps.

Unless the deflocculated high solids clay pulp was prepared in agitated equipment provided with an impeller capable of rotating at a high speed simultaneously while beating air therein, the pulp should be transferred to such equipment prior to conditioning with the anionic carboxylic reagent. Before adding the anionic reagent, the pH of the pulp may be adjusted to a value within the range of about 8 to 9 or above, if necessary, before the reagent is added. Prior to the addition of the anionic reagent, flotation adjuvants such as ammonium sulfate may be added. Suitable anionic collector reagents include, by way of example, tall oils, oleic and linoleic acids, soaps of the aforementioned and calcium petroleum sulfonates. The collector reagent is preferably emulsified with ammonia before addition to the pulp.

The anionic carboxylic reagent is usually employed in amount within the range of about one-half to 5, preferably 1.5 to 3.0 lbs/ton, based on the dry degritted clay. The preferred range is significantly less than the preferred quantity of anionic carboxylic reagent usually used in Ultraflotation. By limiting the quantity of anionic reagent to an amount which suffices to collect the impurities, residual soap which may cause undesirable foaminess in the clay product is avoided. Furthermore, by limiting the quantity of collector reagent we are normally able to obtain brighter products than if we used quantities suitable for Ultraflotation. When processing unfractionated crudes, the use of trace amounts of collector avoids undesired coating of the larger clay particles.

In addition to fatty acid collector, we prefer to add a small amount of a neutral hydrocarbon oil, e.g., lube oil or fuel oil, to the pulp during the conditioning step. From one-half to 3 lbs/ton, preferably 1 to 2 lbs/ton of hydrocarbon oil, is employed. The use of neutral hydrocarbon oil increases the yield of beneficiated clay, apparently by reducing the volume of the waste froth which contains entrained clay.

The addition of materials which will flocculate and- /or thicken the conditioned pulp should be avoided. Solvents for the collector, e.g., naphtha, should be avoided.

Strong frothers, such as pine oil, should not be added to the pulp when high rosin acid tall oils are used as collectors since the presence of a tenacious froth adversely affects clay recovery.

The conditioning step is carried out in equipment containing an impeller system capable of imparting the proper combination of mechanical energy and air en-. trainment. A Fagergren flotation machine is a specific example of commercially available equipment that may be used in the conditioning step. Reference is made to Perrys CHEMICAL ENGINEERS HANDBOOK," 21-73, Fourth Edition, published by McGraw-Hill Book Company, for a diagram and a description of the operation of a suitable model of a Fagergren machine. As shown in the reference, the Fagergren machine provides mechanical agitation and aeration by a rotating impeller on an upright shaft, the rotor being surrounded by a stationary cage (stator) fitting closely around the rotor and providing a mild shearing action. The so-called l l" Fagergren impeller may be used. Some Fagergren machines are supplied with means to induce air into the standpipe surrounding the impeller shaft for injection below the surface of the pulp during a frothing step. The air inlet is preferably closed when used to condition the pulp because of the possibility that undesired frothing may take place.

In prior art clay flotation processes, especially Ultraflotation, a recommended conditioning time after addition of reagents is 17 minutes when using a 2-inch laboratory Fagergren machine operating with a rotor speed of about 2,900 rpm. Power input is about 30 hp. hr./ton. In carrying out the present invention, conditioning time is preferably less than 30 minutes and may be as little as 10 minutes; power input is below hp. hr./ton and is preferably in the range of 10 to 30 hp. hr./ton.

During conditioning the anionic carboxylic reagent selectively coats the colored particles of titaniferous impurity and the coated particles form loosely bonded agglomerates which rapidly rise to the surface of the conditioned body after conditioning has been terminated. The pulp acquires the appearance of a milk shake as conditioning takes place. Immediately after the agitation is terminated, streaks of agglomerated impurity may be observed ifa glass or plastic conditioning tank is used. Provided the pulp is maintained sufficiently nonturbulent, these agglomerates will rapidly rise to the surface of the pulp and form a colored spongy foam or scum. Thus, when the conditioned pulp is maintained under sufficiently quiescent conditions in a transparent vessel, the originally creamy apparently homogeneous system will appear to separate virtually instantaneously into a lower creamy fluid deflocculated layer and an upper brown or yellowish-brown spongy scum. Within a few minutes, e.g., 2 minutes, after the spongy scum forms at the surface of the slip, the appearance of the scum begins to; undergo a perceptible change. Clay appears to stream downwardly through the sponge and simultaneously the scum consolidates and its height decreases. The color of the spongy layer becomes deeper when these changes occur.

The conditioned pulp must be maintained sufficiently free from strong agitation to collect the reagentized agglomerates at the surface without forming a voluminous froth which will entrain clay. Generally speaking, collection is most rapid and entrainment of clay with the impurities is minimized by permitting the conditioned pulp to stand without any agitation or aeration. Under such conditions, essentially all of the titania agglomerates will report at the surface of the pulp within a minute. The scum could be removed at this point although clay entrainment may be reduced by allowing the scum to remain on the surface of the pulp for an additional period of time, e.g., by maintaining the pulp under static conditions for a time within the range of about 5 to 10 minutes.

The dark floated layer is separated from the deflocculated slip by scraping with suitable blades or the like. Alternatively the slip may be siphoned or otherwise drained from the scum. Devices constructed along the lines of a conventional separatory funnel may be used for quiescent separation.

To improve upon the recovery of clay from the rougher (first separation) step, the froth product may be diluted with a minimal amount of water, reconditioned with mild stirring and a clay slip recovered. This clay may be combined with clay from the rougher purification. The rougher froth may be cleaned one or more times. Normally one cleaning suffices.

A slip of beneficiated unfractionated clay may be fractionated, preferably in a centrifuge, to produce fine and coarse fractions of desired particle size before or after applying the oxidation treatment, depending upon whether or not it is desirable to brighten further the coarse size fraction. Slips of pre-fractionated fine particle size clay may undergo oxidation without further treatment except for a reduction bleach.

Fractionation of processed soft crudes is usually carried out to produce purified kaolin products that contain at least about 75 percent by weight of particles finer than 2 microns equivalent spherical diameter. For example, the fractionation may be controlled to produce a clay product which is 90 percent to 94 percent finer than 2 microns, thus conforming to the size specification of the commercial clay products Spray Satin or Stellar. Products conforming to the size specification of the HT grades are obtained by operating the sizers to produce products containing 78 percent to 82 percent by weight of clay particles finer than 2 microns. When processing hard clay, e.g., gray Georgia kaolin clay, it may be desirable to fractionate to 100 percent minus 2 or 3 microns and at least 90 percent finer than 1 micron.

A beneficiated coarse size fraction is also recovered as a sediment during the classification step.

Various oxidants amenable to use in aqueous media may be used in carrying out our oxidation treatment. Examples include permanganate salts, peroxides such as hydrogen peroxide, ozone, chloride, sodium perchlorate and sodium hypochlorite. The oxidation may be carried out at any pl-I appropriate for the oxidant that is employed. For example, with a permanganate salt or hydrogen peroxide, the slip should be acidified. With ozone, the slip may be alkaline, neutral or acidic. The oxidant must be one which does not leave a permanently colored residue in the clay.

The temperature of the slip during oxidation may be varied over a wide range. Generally, the use of elevated temperatures permits a reduction in the time required. Using potassium permanganate, for example, we recommend the use of l to 2 lbs./ton KMnO, at elevated temperature, e.g., 180F., for 1 hour or the same quantity at room temperature for 24 hours. Ozone has produced satisfactory results in amount of 1 lb./ton. I-Iydrogen peroxide can be used in amount of about lbs/ton. When processing gray kaolins which have not undergone oxidation treatment prior to quiescent flotation. larger quantities of oxidant are used than when processing soft white clays in order to provide for the oxidation of impurities in gray clay.

The oxidation may be carried out with slips of widely varied solids contents. We prefer to add the oxidant to undiluted slips of beneficiated clay.

After oxidation, the slip should be acidified if necessary to a pH level appropriate for the reducing bleach treatment. Generally, the pH should be below 4 during bleaching. Any of the conventional clay reducing agents may be used. Sodium hydrosulfite and zinc hydrosulfite are recommended. Generally from 6 to 12 lbs./ton of a hydrosulfite salt will suffice when a permanganate salt is employed as the oxidant in amount within the range of l to 2 lbs/ton.

After bleaching, the clay is filtered and washed in conventional manner. The washed clay may be dried or prepared into a high solids slurry suitable for shipment.

EXAMPLES The overall objective of the tests to be described was to produce by quiescent flotation and bleaching No. l coating fractions having a brightness of at least percent, preferably 91 percent or more, under the most economical conditions, including power requirements and equipment costs.

The clays used in all examples were typical of the Georgia Fall-line soft kaolin crudes mined and wet processed to produce No. l coating clays. The clay was of the type which is brightened significantly by zinc hydrosulfite bleaching but not by oxidation treatment. The clay contained yellowish titania as am impurity.

In all cases brightness was measured by TAPPI standard method T-646 m54.

All quantities are reported on a weight basis. Reagents are reported on the basis of reagent percent active ingredient) per ton of dry clay.

EXAMPLE I This example illustrates the effect of varying power input during conditioning on the quiescent flotation of titania from a pulp of Georgia kaolin clay. In this example, and in Example II, conditioning tests were conducted using a laboratory Fagergren mixer (2.0 inch rotor and 3% inch stator) in a 7% inch tank having a capacity suitable for handling a l,250 gram clay charge (1 gallon). The mixer was equipped with a watt meter so that power consumption could be measured. The rotor was operated at 2,900 rpm, corresponding to a peripheral speed of 1,520 ft./min., for the time indicated in the examples.

The crude undegritted clay was blunged in deionized water at 40 percent solids for 10 minutes in a Denver agitator. Soda ash was added in amount of 3 lbs./ton and agitation was continued for 15 minutes. The pulp was deflocculated by adding a dilute hydrosol containing 5 lbs/ton N sodium silicate solution (38 percent solids) and 0.5 lbs./ton alum. The slip was then agitated for 10 minutes and degritted on a 325 mesh screen.

In the tests, degritted pulps containing 1,250 grams of clay (dry basis) at about 33 percent solids were reagentized with the following reagents added in the order listed: ammonium sulfate, 3.0 lbs./ton, added as a 5 percent solution; an emulsion of collector reagent obtained by agitating ammonium hydroxide, 0.2 lb./ton, as a 2.5 percent solution with crude tall oil acids (M-28), 2.0 lbs/ton, and Eureka M oil, 1.0 lbs/ton. The reagentized pulps were conditioned for 15, 30 or 60 minutes in the 2 inch Fagergren conditioner. The pH was about 8.6 during conditioning. After the conditioning step, the pulps were transferred to large separatory funnels in which they were allowed to stand untouched and without being moved for about 6 minutes. In all cases, the conditioned pulps separated into two layers includinga dark spongy scum and a lighter colored slip of deflocculated clay. The slips below the floated layers were withdrawn as underflows. The slips of beneficiated clay were fractionated and No. 1 coating fractions (at least 90 percent by weight of particles finer than 2.0 microns) were recovered in the form of aqueous slips. The fine size fractions were flocced with sulfuric acid to a pH of 3.0 and portions were bleached with varying amounts of zinc hydrosulfite. Results are summarized in Table I. In the table, the results for bleached brightness represent values obtained with an optimum amount of zinc hydrosulfite (maximum bleached brightness).

TABLE I EFFECT OF CONDITIONING POWER INPUT AND TIME ON BENEFICIATION OF KAOLIN CLAY BY OUIESCENT Results for 2" Laboratory Fagergren Mixer Data in Table I for kaolin beneficiated by quiescent flotation show that increase of conditioning time and power resulted in increases in unbleached and bleached brightness. The data show that a product having 90 percent bleached brightness could not be produced when employing a minute conditioning with the 2 inch impeller (28 hp. hr./ton power input). However, by using conditioning times of 30 minutes or more with power inputs above 50 hp. hr./ton, products having bleached brightnesses appreciably above 90 percent were obtained. To obtain a 91 percent brightness the use of reduced conditioning times and power inputs in the quiescent clay flotation process. Results are shown for Georgia soft kaolin crudes varying considerably in original brightness.

The various crudes were processed by the procedures described in Example I. The 2 inch laboratory mixer was used for conditioning in all cases.

After quiescent flotation, settling, froth removal and fractionation to No. 1 coating clay specifications, the fine size fractions were split into two portions. One portion was acidified to a pH of 3.0 with sulfuric acid and bleached with various amounts of zinc hydrosulfite. In the case of clays beneficiated with low power inputs, portions were acidified to pH 3.0 with sulfuric acid and potassium permanganate was added in amount of 1.5 lb./ton. The slip was mixed at about 180F. for an hour. The oxidized slip was then split into two portions and bleached with various amounts of zinc hydrosulfite.

Results are summarized in Table II. All data in Table II for samples bleached with reduction bleach represent values for optimum amount of ZnS O and thus vary from sample to sample.

Data in Table II for high energy conditioning (40 minutes and 75 hp. hr./ton with the 2 inch impeller) indicate that the crudes of 76.1 percent to 81.6 percent brightness could be beneficiated by quiescent flotation and bleached with zinc hydrosulfite to average brightness values of 92.1 percent, well above the target value of 91.0 percent. In contrast, when conditioning time was reduced to 10 minutes with a 19 hp. hr./ton power input, average bleached brightness of products from the same crudes was only 90.3 percent, significantly below the 91 percent target value.

Data in Table II shows that by treating the beneficiated fraction with only 1.5 lb./ton KMnO before carrying out the reducing bleach, average brightness of the products was 91.9 percent well above the desired value. This result was realized in spite of the fact that with the 10 minute conditioning, average unbleached brightness was only 85.1 percent as compared to 86.5 percent for the 40 minute conditioning time.

TABLE II EFFECT OF POST-OXIDATION TREATMENT ON CONDITIONING* TIME AND POWER REQUIREMENTS ON BRIGHTNESS OF KAOLIN BENEFICIATED BY QUIESCENT FLOTATION Conditioning 10 minutes & 19 hp. hr./ton

(l) Degritted crude (2) Unbleached beneficiated fraction (3) (2) After reduction bleach alone (4) (2) After oxidation treatment and reduction bleach Brightness, 7r

Conditioning 40 minutes & 75 hp. hr./ton

(l) Degritted crude (2) Unbleached beneficiatecl fraction (3) (2) After reduction bleach alone Brightness, 81.6 78 8 76.1 765 Av. 86.5 87 0 85.9 87.6 86.5 91.4 92 5 92.3 92.2 92.1

'2 inches Fagergren Impeller product, a conditioning time of minutes and energy 60 input above 100 hp. hr./ton were required.

The data in Table I therefore show conclusively that optimum product quality was not realized by quiescent flotation of titania from a typical degritted whole Georgia clay crude when mild conditioning was employed.

EXAMPLE ll Tests were carried out to illustrate how the postflotation oxidation treatment of the invention permits EXAMPLE III TABLE III having a brightness of at least 90 percent which comprises the steps of forming a fluid aqueous slip of impure kaolin clay containing finely divided colored particles of a titaniferous impurity, said impure clay having been obtained from a sedimentary soft kaolin clay which is brightened significantly by bleaching with zinc EFFECT OF POST-OXIDATION TREATMENT ON CONDITIONING TIME AND POWER REQUIREMENTS ON BRIGHTNESS OF KAOLIN BENEFICIATED BY QUIESCENT FLOTATION Conditioning minutes & 12.5 hp. hr.lton

(l) Unbleached beneficiated fraction 83.8 (2) (1) After reduction bleach alone 900 (3) (1) After oxidation treatment 9l.0 and reduction bleach Conditioning 120 minutes 8L, hp. hr./ton

(l) Unbleached beneficiated fraction 87.5 (2) (1) After reduction bleach alone 912 Brightness,

Brightness,

' 7 inches Fugcrgrcn Impeller An overall comparison of data in Tables II and III show that when conditioning was carried out in a large scale unit there was an even greater difference between the unbleached brightness with mild and high energy conditioning than when the laboratory conditioner was used. Thus, with the laboratory unit the average difference in unbleached brightnesses with mild and high energy conditioning was 1.4 percent (86.5 to 85.1). In the larger unit, the average difference in unbleached brightness was 5.7 percent (88.6 to 82.9).

The data in Table III show, however, that this significant difference in clay brightness resulting from mild and intense conditioning was essentially equalized, as it was with the smaller laboratory unit, by oxidizing the beneficiated clay with a small amount of potassium permanganate before subjecting the clay to the reduction bleach. Thus, beneficiated clay products having average brightnesses of 90.6 percent were produced with a 12.5 hp. hr./ton power input in 30 minutes and postflotation oxidation treatment whereas the products averaged 90.9 percent bleached brightness when the power input was 50 hp. hr./ton in 120 minutes and no oxidation treatment was employed. In effect, by using the oxidation treatment (1.5 lb./ton KMnO after quiescent flotation, we were able to obtain desired product brightness while carrying out the conditioning with 25 percent of the power input in 25 percent of the time which would have been required if we had omitted the oxidation treatment.

We claim:

1. In a method for producing a kaolin clay pigment hydrosulfite solution but is not brightened significantly by treatment with ozone or other strong colorless oxidants, without adding a froth producing reagent, conditioning said slip by means of a rotating impeller with reagents consisting essentially of an anionic collector capable of selectively coating the particles of titaniferous impurity, maintaining said conditioned slip quiescent until coated particles of titaniferous impurity rise and float on the surface of the slip of clay, removing the floated particles from the slip, thereby obtaining a partially brightened slip of unbleached purified clay, and bleaching the slip of purified clay with a reducing bleach; the improvement which comprises: using an anionic collector in amount within the range of about 1.5 to 3.0 lbs./ton of clay, conditioning the slip for less than 30 minutes with an energy input less than hp. hr./ton, and oxidizing the slip of purified clay with an oxidizing agent which does not leave permanently colored residues in the slip before bleaching it with a reducing bleach.

2. The method of claim 1 wherein the energy input is less than 30 hp. hr./ton.

3. The method of claim 2 wherein the slip of impure clay is conditioned with emulsified tall oil in amount within the range of about 1.5 to 3.0 lbs/ton and from 1 to 2 lbs/ton of a neutral hydrocarbon oil.

4. The method of claim 1 wherein the slip of purified clay is oxidized by treatment with l to 3 lbs/ton KMnO at an acidic pH. 

2. The method of claim 1 wherein the energy input is less than 30 hp. hr./ton.
 3. The method of claim 2 wherein the slip of impure clay is conditioned with emulsified tall oil in amount within the range of about 1.5 to 3.0 lbs./ton and from 1 to 2 lbs./ton of a neutral hydrocarbon oil.
 4. The method of claim 1 wherein the slip of purified clay is oxidized by treatment with 1 to 3 lbs./ton KMnO4 at an acidic pH. 